Endocytosis Flashcards
What is taken up in a cell
Nutrients
Signals
Antibodies
Enzymes
Viruses
Bacteria
Membrane
Endocytosed material can have different fates
Can prolong signalling in cell y recycling it
Material take by cell will need to be handled differently which is why there are three different actions:
- Recycling
- Transcytosis
- Degradation
There are multiple endocytic pathways
Clathrin
Caveolar
RhoA
Flotillin
Cdc42
Arf6
Micropinocytosis
Phagocytosis
Phagocytosis, similar to micropinocytosis, used in to remove apoptotic cells or take up bacteria/viral cells
Phagocytosis: uptake of large particles (Bacteria, apoptotic cells)
Pathogens are often coated by antibodies, called opsonization
Once bacteria stuck to surface receptor, pseudopods form and engulf the bacteria
Actin binding protein pulls phagocyte into cell after pseudopods bind
Frustrated phagocytosis
When two different phagocytes try to engulf one bacterial cell they get frustrated as they cannot bind to each other as are different cells
Fusion of ruffles is cell autonomous
First hint membrane recycling occurs
Macrophages induced to phagocytosed of 1.1µm latex beads
By counting beads was possible to estimate how much membrane was internalised
Result; 305 total SA internalised per hour but no change to cell size so therefore membrane must be recycled as cannot be synthesised at that rate
Macropinocytosis
Cells form actin driven ruffles which sometimes fuse to form macropinosomes which are internalised within the cell, which is much larger in size that vesicles
Happening in a lot of cells in our immune system so that it can sample the environment regularly
Mechanistically similar to phagocytosis
Non selective uptake of extracellular material
Used by cancer cells to take up nutrients
Clathrin-mediated endocytosis
Is pathway we know most about, originally identified in 70’s
Based on uptake low density microprotein, microproteins can transport cholesterol around body
Steps:
1. LDL will go to a LDL receptor that’s in the clathrin coated pit which then coats the LDL and the receptor
2. This then has to be uncoated one inside the cell to fuse with the endosome
3. The LDL is then targeted to lysosomes so the LDL can be broken down and the cholesterol taken out of the centre
4. The LDL receptors are then recycled and returned to the the cell surface
In between the cargo and the coat of the vesicle are adapter molecules
This all happens in about 20 minutes
Most of what we know can from Brown and Goldstein and they are interested in hypercholesterolemia
Hypercholesterolemia - Mutations in LDL receptors: defective binding of LDL or defective internalisation
Calthrin coated pits and vesicles are very regular lattices
Clathrin triskelia polymerise into lattices
When purified can see clathrin and will come together into lattice
Consists both heavy and light clathrin chains, both important
Only need remember it is three legged molecule
Lattice forms based on biochemical information
Dynamin required to pinch off clathrin coated vesicles
Dynamin protein first noticed in Drosophila
Temperature sensitive mutants of shibire mutants in Drosophila undergo reversible paralysis when they get to certain temp, but one temp lowered again they recover
Discovered that when temp get high is build up of clathrin coated vesicles at membrane that haven’t broken off
Showed that mutation linked to protein responsible for final pinching off of clathrin coated vesicles
Gradient pH key for function on endocytic pathway
Rab proteins deine intacellular organelles
Rabs facilitate … (go back over)
OVerexpression of rab5 results in enlarged endosomes
Cargo for degradation is incorporated into intraluminal vesicles (ILVs)
Late endosome: Multivesicular body (MVB)
When materials targeted to lysosomes, hydrolases and lipases break down material which will then get used throughout cell
Lysosomes can be stained under electron microscope
